1. Field of the Invention
The present invention is in the field of lighting control. More particularly, the present invention pertains to apparatus and methods for controlling the voltage applied to a lighting unit, such as a HID (high intensity discharge) lamp. Such HID lighting advantageously provides a high light output per watt of energy consumed. Further, once started at full voltage, such HID lighting may sustain operation at a substantially reduced voltage with little or no reduction of light output, but with a substantially reduced energy consumption.
2. Related Technology
The present invention constitutes an improvement over prior art U.S. Pat. No. 4,431,948, issued Feb. 14, 1984, and also over U.S. Pat. Nos. 5,528,110; 5,508,589; and 5,623,186, the disclosures of which are incorporated herein by reference to the extent necessary for a full disclosure and understanding of the present invention.
The '948 patent discloses a controller for HID lighting effecting a selective reduction of applied voltage by use of an autotransformer connected in series with a switch. A first portion of the autotransformer winding is interposed between the input and the output, and an additional winding portion is interposed between the output and the common or neutral terminal of an alternating current power source. The first portion of winding is referred to the series winding of the autotransformer, and the second portion of the winding is referred to as the common winding of the autotransformer. The switch (which may be implemented as a relay, for example), is connected in series with the common winding so that when the relay is in its open condition, no current flows in the common winding, and the output voltage is substantially equivalent to the input voltage.
On the other hand, according to the '948 patent, when the switch contacts of the relay are closed current is permitted to flow in the common winding and the autotransformer performs its normal function with the output voltage reduced relative to the input voltage. However, the invention of the '948 patent has significant deficiencies, as is pointed out in the three later patents identified above.
In the three later patents identified above, an inventive autotransformer is provided with a bucking coil in order to cancel the creation of harmonics and inadvertent heating affects in the autotransformer. This inventive autotransformer avoids the deficiencies of the '948 patent. However, further improvements to the technology for controlling HID lighting are possible. There is, therefore, a need to provide an improved apparatus and method for control of HID lighting in order to achieve lowered power consumption.